Deciphering the "conversation" between bacterium and host during infection is central to understanding microbial pathogenesis and host defense. Periodontal diseases have long been recognized as bacterial infections, and a long-term goal of the laboratory is to identify P. gingivalis genes expressed during infection, ie. one side of the cross-talk between bacterium and host. The genetic tractability of the organism, combined with the availability of a completed genome sequence and gene microarrays, has made this goal approachable. Bacteria use two-component signal transduction systems to monitor and react to changes in their environment. In this application, we test the hypothesis that in P. gingivalis these systems are responsive to in vivo conditions prevailing in the subgingival crevice during health and disease. A comprehensive study is proposed of the molecular mechanisms of gene regulation in P. gingivalis, a new area of research with this organism. Response regulators will be analyzed to identify the genes they control, and their promoter targets. We will test our hypothesis by determining the expression of a subset of identified genes in the in vivo environment of subgingival plaque from periodontitis patients. The experimental approaches exploit our experience in molecular genetics, whole genome transcription profiling, promoter target localization, and DNA sequence analysis. The goals are responsive to the RFA since we will analyze P. gingivalis intracellular signaling mechanisms that orchestrate responses to environmental changes. By defining the regulators, the genes they control, and their in vitro and in vivo expression patterns, we can identify conditions that may trigger periodontitis, and target rational therapeutic interventions to the regulatory molecular switches.